1. Field of the Invention
The present invention relates to a planar light source device. More particularly, the present invention relates to a planar light source device providing uniform brightness and a display using the same.
2. Description of the Related Art
Conventionally, backlight units for liquid crystal displays (LCDs) have been planar light sources. Such planar light sources include an edge light type using an edge emitter with a light guide plate (LGP) and a direct light type using an array of light sources.
In the edge light type source, white dots or lines may be formed on a rear surface of the light guide plate in order to direct light to a light crystal panel (LCP) by guiding the light along the LGP using internal reflection. In order to output illumination having in-plane uniform brightness, the edge light type source also needs a diffusion sheet to blur a pattern formed by specular reflection off the rear surface.
Direct light type sources may also include a diffusion plate and a diffusion sheet disposed on an upper surface of the light source device in order to obtain in-plane uniform brightness by blurring a pattern of the array of light sources. Direct light type sources may include cold cathode fluorescent lamps (CCFLs) outputting white light and light emitting diodes (LEDs) emitting multiple wavelengths, e.g., red, green and blue, which combine to form white light.
The diffusion plate and/or the diffusion sheet used for the conventional planar light source devices has a strong light diffusion function in order to conceal an underlying pattern and to prevent regions of nonuniform brightness called mura. The conventional planar light source devices have a problem in that light is diffused in directions other than the direction of the LCP, thereby lowering the brightness of the LCD.
One proposed solution includes passing light emitted from a rod-shaped light source through a triangular prism having angled faces in a longitudinal direction of the rod-shaped light source. The triangular prism produces virtual images in addition to the real image, thus producing illumination having in-plane uniform brightness without reducing brightness of the light source. However, the triangular prism has a simple cross-section with two angled faces serving as refractive surfaces. Since only two virtual images are formed for each input light, the triangular prism still cannot provide a desired brightness or prevent generation of mura.